Avogadro's number
The count of molecules in one mole, 6.022 × 10²³ — the bridge between the gram-and-litre world and the molecule-by-molecule world.
Definition
Avogadro's number, N_A = 6.02214076 × 10²³ per mole, is the number of elementary entities (atoms, molecules, ions) in one mole of a substance. Since the 2019 revision of the SI it is a defined exact constant, and the mole is defined as exactly that many entities. It converts macroscopic amounts measured on a balance into microscopic counts of particles.
The number is named for Amedeo Avogadro, whose 1811 hypothesis — that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules — implied that such a universal count must exist, though Avogadro never determined its value. It links the gas constant and the Boltzmann constant through R = N_A k_B.
The first reliable measurements came a century later. Jean Perrin's 1908 study of Brownian motion yielded N_A independently of any chemical method, and his agreement with values from electrolysis, blackbody radiation and other routes was decisive evidence that atoms are real and countable. Perrin proposed naming the constant after Avogadro.